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Topic hub — 2026-09-19

Bitterness in Southeast Asia

Thai public guides to vegetables carry a category called rot khom, bitter taste, and list khilek flowers and pheka pods inside it. Bitterness is used as a classifying word at the same level as sweetness or heat.

Most of the material gathered here, though, does not measure bitterness. What it writes down is how to take it out — and those instructions come not in temperature or in minutes but in counts: boil and discard at least twice; three to five minutes, depending on how bitter you want it. This page sets the greens that are called bitter beside the few measurements that carry numbers, without closing the distance between them.

What this page establishes

• Thai public vegetable guides carry a bitter-taste category, with khilek flowers and pheka pods given as examples.
• The removing is written in counts and in preference: khilek is boiled at least twice in boiling water and rinsed in fresh water; sadao is blanched for 3 to 5 minutes with the time adjusted to how bitter you want it.
• The one case with a measured bitterness figure is mee soto broth. Against a control bitterness intensity of 5.94, cutting NaCl by 22% and 31% gave 5.60 and 5.63, with no significant difference.
• What did significantly lower the bitterness in that broth was cutting NaCl by 40% and adding flavour enhancers at 0.20% and 0.40% (4.33 to 4.85).
• Some greens in this bundle are not called bitter at all. The pharmacopoeial monograph for phak phai, 11 pages long, never uses the word bitter; its taste is recorded as slightly tingling.

Bitterness as a category of vegetable

Start with bitterness as a category. An article from the Highland Research and Development Institute sorts vegetables by taste and gives pheka, the pod of Oroxylum indicum, as an example of the bitter-taste category.

The flowers of khilek (Senna siamea) sit in the same category. A shelf labelled "bitter vegetables" already exists on the guidance side.

Item-level entries record bitterness too. For rau dang, a public vegetable database states that shoots and young leaves have a bitter taste. One line, with no intensity and no method.

In ethnobotanical compilations, taste and aroma sit in separate columns. The leaves of Melicope pteleifolia are recorded with taste bitter and aroma citrus, used by the Karen to add aromatic essence to chili paste or spicy soup. The record carries the observation label.

The same table records the fruits as bitter with a citrus aroma, used by the Thai Yuan to scent spicy meat soup. Bitterness here is not a fault; it comes along with the aroma being used.

The removing is written in counts

What sits on the bitter shelf comes with a removing step. Senna siamea is recorded as a wild food plant under the local name khilek, heated before use to reduce its bitterness.

How that heating is written is what characterises the region. Boil in boiling water at least twice, sometimes with a little salt added, then rinse in fresh water. Not a temperature but a count; not an endpoint but a sequence.

The dish side reads the same. For gaeng khilek, the leaves and flowers are boiled and the water discarded, then boiled again until soft and squeezed out.

Whatever bitterness survives is then handed to another ingredient. A university recipe's note states that khilek is bitter, that the water is discarded several times, and that coconut milk cuts the bitterness. Take it out, then cover what remains.

For sadao — the young shoots and inflorescences of neem — three methods are listed side by side: blanch, simmer in boiling rice-rinsing water, or wilt it over the fire, all to reduce the bitterness.

And the timing has preference built into it: blanch in hot water for about 3 to 5 minutes, adjusting the time to how bitter you want it, then into cold water. Inside that sentence, the one who decides how far to take it out is the person eating.

The balance is struck at the end by what is served with it. The sadao shoots and flowers come with a sweet fish-sauce dip.

Sensations that are not bitterness

Several greens gathered in this bundle are described by a sensation other than bitterness. Rather than drop them, what the sources actually say is kept.

Phak phai (Persicaria odorata) is included in the Thai Herbal Pharmacopoeia as a crude drug of the aerial parts.

That monograph describes the odour as aromatic and characteristic and the taste as slightly tingling, and across all 11 pages the word bitter never appears. This is exactly why this site keeps pungent and bitter apart as a crossing.

Phak khayaeng (Limnophila aromatica) is the same. Broken stems and leaves are recorded as strongly fragrant and pungent. Not bitter.

Its use is explained from the aroma side: eaten raw alongside larb and nam phrik, or used to cut off-odours and add fragrance to soups. It sits at the same table as the bitter greens while carrying a different sensation.

The one case with numbers

Only one source on this page measures bitterness on a scale, and it is not a study of traditional cooking. It is a study of salt reduction.

In mee soto broth, adding MSG or an MSG-nucleotide flavour enhancer at 0.20% and 0.40% to a reduced-salt formulation raised umami, chicken flavour, mouthfulness and sweetness against the control and lowered sourness and bitterness. The panel was 24.

Look closer and cutting the salt alone did not move the bitterness. Against a control bitterness intensity of 5.94, the 22% NaCl reduction gave 5.60 and the 31% reduction 5.63, neither significantly different. Bitterness fell significantly only in the 40% reduction with enhancer added (4.33 to 4.85).

People often say that cutting salt makes food taste bitter. In this measurement it did not. What moved things was what was added.

Bitter greens as companions

Some entries do carry compositional figures. Extracts of pheka pods contain baicalin and baicalein, quantified at 2.57% w/w baicalein in short-stalk pods and 0.89% w/w in long-stalk ones. That quantification is not a measurement of bitterness.

The pods themselves are roasted and eaten as a local vegetable, available in markets in northeastern Thailand.

For Tacca chantrieri, the flowers and young leaves are recorded as grilled over a fire or blanched before eating.

And the eating is written as a combination rather than a dish on its own: villagers grill or blanch the flowers and young leaves to eat with nam phrik and with larb.

Larb itself is the name of a method, not of an ingredient: finely mincing meat and seasoning it with a chili-paste mix called phrik larb. The bitter greens appear as what is served alongside it.

Thua paep (Dolichos lablab) occupies the same position. The young pods are boiled and eaten alongside chili paste, fermented fish and sour fish dishes.

Questions people ask

How bitter are Southeast Asian bitter vegetables?

Within this page there is almost no record of intensity. Bitterness is used as a classifying word and written into item entries in a single line. The only thing measured on a scale is mee soto broth, and that comes from a salt-reduction study rather than from traditional cooking.

How many times should the water be discarded?

The sources write counts. Khilek: boil at least twice in boiling water and rinse in fresh water. Gaeng khilek: boil, discard the water, boil again until soft, squeeze. Sadao: 3 to 5 minutes, adjusting the time to how bitter you want it. As that last line shows, the endpoint is set by the eater, not by the source.

Are all these greens bitter?

No. The pharmacopoeial monograph for phak phai records the taste as slightly tingling and never uses the word bitter across 11 pages. Phak khayaeng is described as pungent and strongly fragrant. They share a plate with the bitter greens while carrying a different sensation.

Does cutting salt make food taste bitter?

Not in mee soto broth. Against a control bitterness of 5.94, cutting NaCl by 22% gave 5.60 and by 31% gave 5.63, neither significantly different. Bitterness fell significantly only in the 40% reduction with 0.20% or 0.40% enhancer added.

What is not known

• Most entries in this bundle have no record of measured bitterness intensity. They are written as bitter; how bitter is unknown.
• Nothing measures the relation between the number of boil-and-discard cycles and the bitterness that remains. What differs between two and three is not written.
• The statement that coconut milk cuts the bitterness is a recipe note, not a measurement.
• The baicalein figures for pheka pods (2.57% and 0.89% w/w) are compositional measurements with no correspondence to bitterness shown.
• The mee soto result comes from a salt-reduction study with a panel of 24, a single study that does not extend to other dishes.
• The bitter entry for Melicope pteleifolia carries the observation label from a compiled table, with no panel and no scale.

Read in the encyclopedia

• Khilek (Senna siamea) — flowers on the bitter shelf, boiled and discarded at least twice
• Gaeng khilek — take it out, then cover what remains
• Neem young shoots (sadao) — 3 to 5 minutes, depending on how bitter you want it
• Sadao nam pla wan — balanced by a sweet fish-sauce dip
• Pheka (Oroxylum indicum) — on the bitter shelf, with only its compounds quantified
• Phak phai — recorded by the pharmacopoeia as slightly tingling, never as bitter
• Phak khayaeng — described as pungent and fragrant
• Mee soto broth — the one source here where bitterness was measured on a scale
• Larb — the name of a method, not an ingredient; where bitter greens are served alongside

Go deeper in the research notes

• Eating bitter leaves — ways of eating leafy greens around the world
• Aku-nuki and unmeasured bitterness — what to do with a process whose endpoint is never written
• Bitter, astringent, pungent — separating sensations that share a plate
• From plant parts to bitter seasonings — how a bitter green becomes a spice

Source ledger

The primary sources and limitations for the claims on this page can be checked from the source links inside each card. The full list is on the Japanesesource ledger.